Solar cells are devices that convert the energy of light into electricity using the photovoltaic effect. Solar power is an attractive green energy source because it is sustainable and produces only non-polluting by-products. Accordingly, a great deal of research is currently being devoted to developing solar cells with enhanced efficiency while continuously lowering material and manufacturing costs. When light hits a solar cell, a fraction of the incident light is reflected by the surface and the remainder transmitted into the solar cell. The transmitted photons are absorbed by the solar cell, which is usually made of a semiconducting material, such as silicon which is often doped appropriately. The absorbed photon energy excites electrons of the semiconducting material, generating electron-hole pairs. These electron-hole pairs are then separated by p-n junctions and collected by conductive electrodes on the solar cell surfaces. FIG. 1 shows a minimal construction for a simple solar cell.
Solar cells are very commonly based on silicon, often in the form of a Si wafer. Here, a p-n junction is commonly prepared either by providing an n-type doped Si substrate and applying a p-type doped layer to one face or by providing a p-type doped Si substrate and applying an n-type doped layer to one face to give in both cases a so called p-n junction. The face with the applied layer of dopant generally acts as the front face of the cell, the opposite side of the Si with the original dopant acting as the back face. Both n-type and p-type solar cells are possible and have been exploited industrially. Cells designed to harness light incident on both faces are also possible, but their use has been less extensively harnessed.
In order to allow incident light on the front face of the solar cell to enter and be absorbed, the front electrode is commonly arranged in two sets of perpendicular lines known as “fingers” and “bus bars” respectively. The fingers form an electrical contact with the front face and bus bars link these fingers to allow charge to be drawn off effectively to the external circuit. It is common for this arrangement of fingers and bus bars to be applied in the form of an electro-conductive paste which is fired to give solid electrode bodies. A back electrode is also often applied in the form of an electro-conductive paste which is then fired to give a solid electrode body. A typical electro-conductive paste contains metallic particles, glass frit, and an organic vehicle.
Recently, it has been found that solar cells based on wafers with high sheet resistance, often with a sheet resistance above 80 Ohm/sq., so called high Ohmic wafers, have the potential for increased cell performance. However, disadvantages exist in connection with the use of high Ohmic wafers for producing solar cells, particularly in the form of high contact resistance of the contact between such wafers and electrodes.
There is thus a need in the state of the art for improvements to the approach to producing solar cells from high Ohmic wafers.